Application specific LSI (ASIC: Application Specific Integrated Circuits) are provided with a plurality of logic cells, which are basic logic circuits, and the logic cells are selected for operation according to the user's specifications. ASIC include both conventional ASIC in which the logic cells are determined in an interconnect patterning step during the process of fabrication, and programmable logic in which logic cells are determined by the user after shipment of the product (reconfigurable logic integrated circuits). Although conventional ASIC have the merit of low unit product price achieved by mass production, they also suffer from the demerits that their development is both expensive and time-consuming. Programmable logic, on the other hand, are more expensive per unit than conventional ASIC, have slower operation speed, and consume more power, but are advantageous in that their development is inexpensive and takes little time. Development is now in progress to both reduce the size of programming switching elements and improve the operation performance in order to reduce the product unit cost, increase the operating speed, and reduce the power consumption of programmable logic.
FIG. 1 is a schematic view showing an example of the configuration of programmable logic of the related art.
Programmable logic 110 as shown in FIG. 1 is made up from: a multiplicity of logic cells 112 arranged in a two-dimensional array; interconnects for interconnecting the logic cells; and a multiplicity of switches 114 for switching between connection and non-connection between interconnects. Changing the connection states (connection/non-connection) of two-terminal switches sets the configuration of interconnects between logic cells and the functions of the logic cells to enable the realization of a logic integrated circuit that meets specifications. In the following explanation of the background art, the programmable device disclosed in JP-A-2002-536840 is used in switch 114. In the following explanation, a programmable device is referred to as a two-terminal switch.
A two-terminal switch includes: an ion conduction layer and a first electrode and second electrode arranged with the ion conduction layer interposed. The ion conduction layer is a conductive medium for metal ions supplied from the second electrode. When a negative voltage is applied to the first electrode relative to the second electrode, metal precipitates at the portion of ion conduction layer that contacts the first electrode and grows toward the second electrode, whereby the first electrode and second electrode are connected by the precipitate metal. This is the ON state. Conversely, when a positive voltage is applied to the first electrode relative to the second electrode, the metal that has precipitated dissolves in the ion conduction layer to cut the connection between the first electrode and the second electrode. This is the OFF state. In the following explanation, the voltage that is applied to the first electrode for the making the transition from the ON state to the OFF state or from the OFF state to the ON state is referred to as the switching voltage.
The first electrode of this two-terminal switch is connected to signal line 116 in programmable logic 110, and the second electrode is connected to logic cell 112. A two-terminal switch that has been set to the ON state by the user's operation maintains the state in which the first electrode and second electrode are electrically connected. When a logic signal reaches the first electrode by way of signal line 116, the logic signal is received in logic cell 112 by way of the second electrode.
A two-terminal switch that has been set to the OFF state maintains the state in which the electrical connection between the first electrode and second electrode is cut. In this case, even when a logic signal arrives at the first electrode by way of signal line 116, the logic signal cannot be received in logic cell 112 that is connected to the second electrode.
Thus, in programmable logic, a two-terminal switch that has been set to the ON state by the user functions as a signal line, and a logic cell that is connected to a two-terminal switch in the ON state is maintained in an operable state.
On the other hand, the present inventors are developing a three-terminal switch in which this two-terminal switch is equipped with a third electrode that serves as a control electrode for switching states. In a three-terminal switch, the metal that connects the first electrode and the second electrode can be made thicker in the ON state and the resistance between the first electrode and the second electrode can be made smaller than for a two-terminal switch.